When particles of mineral ore or powder mixtures are sufficiently large, for example larger than 325 (U.S.)mesh, the components of the mixture can be separated by simple physical means such as air or magnetic separation. When the particles in the mixture are finer, more sophisticated technology may be needed to bring about efficient separations. It is conventional to make the separation of finely divided mineral, e.g., particles finer than 325 mesh (U.S. Sieve), by forming the mixture into an aqueous pulp and adding chemicals that will bring about the desired separation. One widely used procedure is froth flotation. In the case of froth flotation of phosphate or oxidized minerals from siliceous gangue, it is conventional to use a fatty acid collector and a salt promoter. The collector coated minerals particles are separated from gangue in the form of a froth. A frother and aeration are frequently employed. When froth flotation is applied to extremely finely divided (slimed) minerals such as certain kaolin clays, froth flotation of the colored impurities in the clay using a fatty acid collector becomes more difficult and it is necessary to use a clay dispersant to keep the clay particles dispersed during froth flotation. A seminal event in the flotation beneficiation of slimed minerals, especially the flotation of colored titaniferous impurities from fine particle size kaolin, is described in U.S. Pat. No. 2,990,958 Green, et al. This procedure is frequently referred to in the art as ULTRAFLOTATION. ULTRAFLOTATION has been practiced on a vast scale for several decades to upgrade kaolin clays. The process has been extended to the beneficiation of other commercially valuable minerals such as cassiterite (tin oxide), phosphate slime, fluorite and other nonsulfide minerals. Another commercial kaolin flotation process, referred to as TREP, employs calcium chloride and oleic acid. See U.S. Pat. No. 4,472,271, Bacon, et al. In the case of kaolin clays which contain significant amount of slimes, conventional froth flotation techniques may not produce the desired removal of colored bodies.
So-called "selective flocculation" is another procedure that is widely used commercially to separate finely divided minerals and powders. In the case of clay, some procedures utilize anionic polymers to selectively flocculate the clay, leaving the impurities dispersed and amenable to subsequent separation. Commercial variants of selective flocculation employ weakly anionic polymers such as hydrolyzed polyacrylamide to selectively flocculate impurities in the clay, leaving the purified clay dispersed. See, for example, U.S. Pat. No. 3,837,482, Sheridan, U.S. Pat. No. 3,701,417 and U.S. Pat. No. 3,862,027, both Mercade, U.S. Pat. No. 3,371,988, Maynard, et al., and U.S. Pat. No. 4,604,369 (Shi).
Early in the history of froth flotation it was proposed to add an anionic polymeric flocculating agent to a mineral pulp already conditioned with a fatty acid collector. This was followed by froth flotation to bring about the separation. See U.S. Pat. No. 3,138,550, Woolery.
To achieve selective adsorption of a flocculating agent on a particular component of a mixture a number of methods have been suggested in the literature [Yu and Attia; in "Flocculation in Biotechnology and Separation Systems," (Y. A. Attia, ed.), p. 601, Elsevier, Amsterdam, 1987 ; Behl, S. and Moudgil, B. M., Minerals and Metallurgical Processing, 5, 92, 1992 and, Behl, S. and Moudgil, B. M., Journal of Colloid Interface Science, 160, 1993]. One of the methods involves selectively blocking the active sites on the inert or nonflocculating component for adsorption of the polymeric flocculating agent. This may be achieved by adsorption of a lower molecular weight fraction of the flocculating agent, which can act as a dispersant and/or site blocking agent prior to exposing the particle surfaces to the flocculating agent.
Both froth flotation and selective flocculation have limitations, especially when applied to slimed ores. In the case of froth flotation of kaolin clay in which a significant portion of the material is in the sub-micron size range, even ULTRAFLOTATION may not bring about adequate separation of color bodies in kaolinite to a commercially viable level at an acceptable recovery of purified kaolin. Similar difficulties are experienced when TREP is utilized to beneficiate such ultrafine clay on a commercial scale. On the other hand, selective flocculation processes utilizing anionic polymers generally result in flocs that are very slow to settle unless copious quantities of salt are used to facilitate sedimentation of the flocs. This necessitates costly multiple washing steps because the presence of salt with the clay would adversely affect the rheology of the clay.